Sap- suckers on the prowl
PROBABLY the most worrying of all plants pests right now are scale insects. There’s a huge range of them and they come in all shapes, sizes and general appearances. These fast- breeding sap- suckers cause a great deal of distress because of the damage they can do and in some cases the mess they create.
For example, citrus trees are highly vulnerable to attacks from soft scale. We can see these as masses of tiny lumps along the youngest shoots and clustered along the midribs of leaves – mainly underneath and out of sight.
They feed constantly on the sap and serious infestations will weaken trees and stunt growth. They produce honey- dew which also attracts ants to feed off it. This sticky substance also falls on leaves, then as it ages becomes an unsightly, black sooty mould that covers leaves and fruit.
Often this is the first indication that a tree has become infested with soft scale and many inexperienced gardeners mistakenly believe that the black- encrusted foliage is the major problem.
A safe means of control is to spray the entire canopy – including beneath all leaves – with diluted white oil emulsion or pest oil. Two thorough sprays at five- day intervals are enough to suffocate the pests, although the sooty mould will persist until gradually washed off by rain or irrigation water.
Other scales have a completely different appearance. Cottony cushion scale is now attacking many apple trees, especially the youngest shoots and leaves. They look like small, well- separated blobs of cotton wool, clinging to the undersides of leaves. Good control can be achieved by pruning out all infested leaves and branches, which would be normal summer pruning anyway. Be sure to rake up and cart away all debris afterwards.
Cottony cushion scale closely resembles another pest known as hydrangea scale. Old, unpruned and neglected hydrangeas, especially those under some kind of stress due to lack of water, are usually the main targets.
The white, cottony scale bodies are clearly visible on all parts of hydrangea bushes – usually mop- tops – including older stems, new shoots and leaves. In fact, a badly infested hydrangea will be dotted with hundreds of these unsightly white blobs.
The obvious treatment to control hydrangea scale is to first water plants so roots are heavily soaked. Then prune out all old, woody branches almost down to the ground. Finally, spray with diluted white oil emulsion or pest oil to kill any remaining pests.
This season has seen an increase in a common apple tree pest that looks a bit like the cottony cushion scale. In fact it is a species of aphid that covers itself with masses of fine, waxy threads that look like white wool – and is in fact called woolly aphid.
When clustered together, woolly aphids look as though someone has stuck masses of cotton wool along the undersides of branches.
This pest tends to concentrate in areas where branches have been wounded by wind or pruning. As they suck the sap, the trees constantly try to repair the damage by producing lumpy, distorting calluses along infested branches.
A quick and easy means of control is to use an old paintbrush dipped in methylated spirit to paint the feeding clusters. This instantly kills the pests, although this can be a tedious job with a large, badly infested tree.
Many varieties of crabapple trees are highly susceptible to woolly aphid attack. Take particular care to paint the area below graft unions on all apple varieties because this stock wood is a common breeding and over-wintering place for these pests.